The present invention relates to a securing arrangement of conductors and, more particularly, to the rigid securing of the stator coil end windings of turbine generators.
The stator coil end windings of a turbine generator typically extend axially from the stator structure for a significant distance. Since the end winding segment of each conductor is not secured within a stator core slot, as its straight segment is, it is subject to severe vibrations and stresses during operation. These effects can be caused by mechanical vibrations during normal operations or the very large electromagnetic forces which occur during abnormal short-circuit or fault conditions.
Normally, each individual conductor is braced to its associated conductors to form a conically shaped end winding assembly which is coaxial with, and extends axially from, the stator structure. Typical turbine generator designs employ two coaxial stator coil assemblies with the straight segments of an inner assembly being disposed radially inward from the outer assembly in the stator core slots. The end winding segments of the inner coil assembly are associated in a conical shape which is radially inward from, and coaxial with, the similarly associated end winding segments of the outer coil assembly.
Bracing and securing the end winding assemblies is conventionally done with conformable pads or blocks located between various support rings and the conductors or between adjacent conductors. The support rings usually extend around the end winding assemblies and the pads typically consist of a resilient material, such as Dacron felt, impregnated with a resin compound. The impregnated pads are placed between the conductors and support rings, at appropriate places, before the resin is cured.
Since the distance between the conductors and support rings varies, pads of various thickness are required and, occasionally, the padding material must be wrapped around a solid block of non-conductive material to accommodate the larger gaps. This variation of gap distance necessitates a large number of different sized pads and complicates the generator's construction.
Although the pads are compressed during implacement, it is difficult to maintain a satisfactory tightness between the support rings and conductors to keep the conductors tight over long periods of operation. A spacing ring or hoop using similar methods as the conformable spacing member is taught in U.S. Pat. No. 3,344,297 issued Sept. 26, 1967 to Bishop, et al.
A significant improvement over the above mentioned technique is disclosed and claimed in U.S. Pat. No. 3,949,257 issued on Apr. 6, 1976 to Cooper et al. It utilizes a flexible hose which is disposed between the support ring and the conductors, extending substantially around the entire circumference of the support ring and filled with resin, under pressure. As the resin is pumped into the hose, the hose assumes a generally circular cross-section between conductors and a flattened cross-section in its segments which are disposed between the support ring and a conductor. This discontinuity of cross-sectioned shape provides bulges between conductors that aid in the prevention of tangential motion of the individual conductors. Another advantage of this invention is that the hose, as the resin is pumped into it, expands radially as much as possible along its entire length. This behavior tends to fill each gap that exists between the conductors and the support ring to the required extent without the need for the custom fitting that is required by previous methods.
Although significant success has been experienced in the application of the Cooper device, some problems have also been discovered. When the resin is completely cured by the application of heat, the coil end windings, support rings and hoses reach equilibrium temperatures as high as 130.degree. C. Although the entire structure is relatively stress free at this temperature, subsequent cooling to room temperature can produce gaps between the conductors, hoses and support rings due to their different coefficients of thermal expansion. The coefficients of thermal expansion for the rings and coils, measured in inches per inch per degree Centigrade (in/in/.degree. C.), are 6.times.10.sup.-6 for the rings and 12.times.10.sup.-6 to 18.times.10.sup.-6 for the coil assemblies, depending on location. The coefficient for the resin-filled hose of the Cooper device is 44.times.10.sup.-6, which accounts for a significant portion of the resultant gaps described above.
Since the coefficient of thermal expansion of the resin-filled hose is a function of the resin composition, a potential solution to the problem was thought to be a resin composition comprising a sufficient proportion of inorganic filler material to reduce the coefficient to a value comparable to that of the coils or rings. This proportion can be calculated to be in the range of 72 to 75 percent of the resin composition. However, this composition becomes a thick paste at room temperature and requires the addition of heat to allow it to be pumped into the hoses. The addition of heat has the deleterious effect of inducing the resin to prematurely cure. Although possible, the success of this method depends on quickly and completely filling the hoses before curing progresses to a point that would prevent the procedure's continuation.
The criticality of time and its resulting uncertainty of success is the primary incentive that induced the present invention which employs a room temperature pumping operation while making possible a reasonably accurate determination of the coefficient of thermal expansion of the resulting resin-filled hose. The method and apparatus of the present invention comprises the insertion of a dry filament subcomposite into the hose prior to its being filled with resin at room temperature followed by a heat curing operation. The coefficient of thermal expansion of the finished hose assembly is determined by the proportions of dry filament materials used and can be varied with reasonable accuracy to produce a filament reinforced, resin-filled hose with a coefficient of thermal expansion comparable to that of the support rings and conductors. The present invention therefore incorporates the advantages of the Cooper hose technique while eliminating its incumbent gap-producing tendencies.